Experiments are being carried out to quantitate effects of negative pi mesons on stem cell inactivation. Comprising a portion of the necessary pretherapeutic work with pions, these studies will provide RBE and OER values for hematopoietic and immunocompetent stem cells. Biologically significant pion depth dose will be determined using a compartmented cell chamber that permits assay of cell inactivation at 3 mm increments throughout the plateau and peak regions.